This invention relates to a silica brick which has a dense structure and an improved thermal conductivity and a method for manufacturing such silica bricks.
Silica bricks are extensively used for the construction of hot blast stoves, open-hearth furnaces and coke ovens.
However, taking silica bricks used in the construction of a coke oven, for example, due to the properties of the quartz included in the raw material, the improvement of the density of the final products (silica bricks) is extremely difficult.
Therefore, even a conventional silica brick of high density which usually is termed as "dense silica brick" or "super duty silica brick" can decrease the porosity to a level of 18 percent at minimum.
Since the coke ovens, in general, are subjected to continuous service over long periods extending up to twenty years or more, the volume stability at high temperatures is of vital importance in the construction of ovens with silica bricks. Therefore, it is important that such silica bricks for the construction of coke ovens include no residual quartz after firing, namely the quartz should be entirely converted into cristobalite or tridymite.
Therefore, such silica bricks must be fired at a temperature higher than the firing temperature of silica bricks for other uses. Such firing however, inevitably rises the thermal expansion rate at firing so that the silica bricks can hardly obtain the porosity of less than 18 percent even when raw mixture has a favorable particle-size distribution.
Along with the demand for enhancement of the productivity of the carbonized material and the NOx regulation which has become very strict lately, the improvement of the quality of the bricks which are used for constructing the wall of coke chambers has become strongly requested these days. Various studies and experiments have been made for that purpose and such studies have developed a method which improves the density and the thermal conductivity of bricks with mere physical addition of metallic oxide such as Cu.sub.2 O, TiO.sub.2 or Fe.sub.2 O.sub.3 and another method which improves the thermal conductivity with physical mixing or addition of a material of high thermal conductivity such as silicon carbide.
However, in the former method, the refractoriness and refractoriness under the load of the silica bricks have both decreased sharply. Thereby, such method has fatal defects in the manufacturing of coke oven bricks. While the final product (the brick) produced by the latter method still contains a considerable amount of SiC after firing. Namely, in the latter method, the SiC amount is intentionally left in the silica brick due to the reason that SiC, which has good thermal conductivity, would improve the thermal conductivity of the entire silica brick. However, the silicon carbide remaining in the silica bricks is oxidized during the service or running of the coke oven and the oxide expands volumetrically giving rise to the deterioration of the inner structure of bricks. Therefore, the bricks produced by this latter method also cause problems under service or running of coke ovens over a long period.
Only aiming at the improvement of the thermal conductivity, several bricks including magnesia bricks, corumdom bricks and corumdom-silicon carbide bricks have been developed. However, when these bricks which generally have a high thermal expansion rate are repeatedly and alternately subjected to heating and cooling, they tend to loosen the brick-made oven construction and thereby they have never been manufactured on a commercial basis.
It is an object of the present invention to provide silica bricks which can resolve the aforementioned defects of conventional bricks wherein the silica bricks of this invention can withstand long use in a hot blast store, an open hearth furnace, a glass furnace as well as a coke oven.
It is another object of the present invention to provide a method for manufacturing such silica bricks.